The IBM Redbook, "PowerVM Best Practices," has a detailed look at mixing vSCSI and NPIV on VIO client LPARs.
From Section 5.1.3:
"It is possible to mix a virtual Small Computer System Interface (SCSI) and N-Port ID Virtualization (NPIV) within the same virtual I/O client. You can have rootvg or booting devices that are mapped via virtual SCSI adapters, and data volumes that are mapped via NPIV.
"Mixing NPIV and a virtual SCSI has advantages and disadvantages, as shown in Table 5-1.
* It makes multipathing software updates easier for data disks.
* You can use the Virtual I/O Server to perform problem determination when virtual I/O clients have booting issues.
* Requires extra management at the Virtual I/O Server level.
* Live Partition Mobility (LPM) is easier with NPIV."
What's your preference? Do you want your SAN guys to provide all your LUNs via NPIV and manage the same multipath drivers on the client for both rootvg and datavg? Or would you rather manage your rootvg multipath drivers on your VIO server, map up the rootvg disks to the clients via vSCSI and use NPIV for your data LUNs?
I prefer to use vSCSI for rootvg. I want to boot my VIO server from my internal disks, map some LUNs to my VIO server to use for my client LPARs rootvg, and then map my data disks via NPIV to my client LPARs. This allows me to troubleshoot by booting my VIO servers locally, and boot my LPARs "locally" via vSCSI.
When I need to update multipath software on the client LPARs, I'm not dealing with a chicken-and-egg dilemma where I'm booting my machine using the same multipath software I now need to update.
When I need to update my client rootvg multipath software, I'm updating my VIO server, which also booted locally. At no time am I "changing the tire while the car is speeding down the road," as might be necessary if I updated drivers when booting my client using NPIV.
Yes, doing it this way requires more effort compared to simply having your SAN team map everything to your clients. In the end though, I believe the benefits outweigh the burdens.
If you disagree, feel free to make your case for NPIV in comments. I'll also accept input from anyone who wants to back me up on vSCSI.